Knowledge of how the human immunodeficiency virus (HIV) fuses with its target cells is important for prevention and treatment of acquired immunodeficiency syndrome (AIDS). Two surface glygoproteins on HIV, gp120 and gp41, interact with CD4 and chemokine receptors on the target cells to initiate the fusion process. In previous reports, we have shown that the amino-terminal fusion peptide of gp41 can serve as a carrier to deliver a peptide probe into cells susceptible to HIV infection. This approach allows one to study which type of receptors are involved. By using different reagents like the chemokines or peptide fragments from gp120 and gp41, one can also study the domains on the receptors involved in the cellular entry of HIV. In the past year, data collected to date on different cell lines with various combinations of receptors and coreceptors using different carriers/probes have been analyzed. The cell lines used include Hela, Vero, NIH3T3, Hos, U87, jurkat, MOLT-3, and 293 cells. Flow cytometry was employed to determine the contents of CXCR4, CCR5, CCR6, and CD4 receptors in each cell line. In addition to the T-tropic gp41 fusion peptide, the M-tropic and simian analogs and a scrambled version of the T-tropic peptide were tested for their ability to deliver cargo into the cells. The findings of our experiment are: 1) gp41 fusion peptide entry into the cell requires the presence of a chemokine receptor. Thus, chemokine receptors are not merely coreceptors for CD4; they are gp41 receptors. However, the presence of gp41 receptor, as is the case with Hos and U87 cells, does not always permit entry. 2) CD4 receptor is not required for entry of the carrier peptide, indicating that the initial gp120-CD4 interaction during fusion is mainly for exposure of the hydrophobic gp41. 3) Entry of gp41 fusion peptide appears not to be affected by chemokines, suggesting that it utilizes a different domain of the receptor. A drug may be designed using the carrier linked to a certain blocker to prevent HIV entry without interfering with the physiological function of the chemokine receptors. 4) The fusion peptide shows no sign of forming higher aggregates as determined by electrospray mass spectroscopy. This seems to suggest that the observed trimerization of gp41 may not occur prior to entering the cell membrane. Trimerization after initial entry may actually provide the necessary energy for virion-cell fusion. 5) The peptide entry requires specific recognition of the amino acid sequence since the scrambled fusion peptide did not work even though the hydrophobicity remains unchanged. 6) The simian fusion peptide, though it differs from the human one, was capable of cellular entry as has been reported by others.